nth-of-type selector

index: The index of each child to match, starting with 1, the string even or odd, or an equation ( eg. :nth-of-type(even), :nth-of-type(4n) )

Because jQuery's implementation of :nth- selectors is strictly derived from the CSS specification, the value of n is "1-indexed", meaning that the counting starts at 1. For other selector expressions such as :eq() or :even jQuery follows JavaScript's "0-indexed" counting.